Green’s Dictionary of Slang

(US, esp. campus/teen) completely, utterly.

1924		E. Ferber ‘Classified’ in One Basket (1947) 228: Whitey’s got it so all over these fat cloak-and-suiters I see you running around with.
1953		K. Tennant Joyful Condemned 103: I am so eighteen.
1972		B. Rodgers Queens’ Vernacular 186: so (adv) very much like, in the same vein as -esque ‘His bathroom is so Esther Williams.’.
1986		C. Stroud Close Pursuit (1988) 172: I’m telling you that my son is so in Jersey!
1995		A. Heckerling Clueless [film script] Excuse me, my shoes! Uh! This is so not fixable.
2003		N.Y. Times 4 May sect. 9 1/1: ‘She was so going places, and he so wasn’t,’ said Kat Stoller, a former housemate at Yale.
2007		L. Redhead Cherry Pie [ebook] ‘Babe [...] I so don’t fancy him’.